the most important. This
place within the central
encompassed by vertically
data manipulation takes
processor (CP) (area
dashed lines). The CP
consists of three basic units. These units are as
follows:
. Control unit—This unit directs the overall
operation of the computer in accordance with a
prescribed plan.
l Arithmetic-logic unit-This unit performs the
actual processing.
. Internal data storage unit-This unit stores the
data to be processed and the prescribed plan
(program).
CONTROL UNIT
In a typical digital computer, the control section
includes the instruction register, the P register, the
general register(s), and the SC register. A brief
explanation of each type of register follows:
l Instruction register—This register holds the
instruction code during execution. The size of the
register is dependent upon the instruction word and
makeup of the computer.
The instruction code
usually has more than one part or field.
. P register—This register contains the address
of the next sequential instruction to be executed. The
contents of the P register are automatically advanced
by one by the P + 1 adder.
l General register—This register stores the
quantity used for address modification. In addition, it
usually has the properties of automatic increment or
decrement. Most computers have more than one
general register.
l SC register—This register consists of one or
two registers to accomplish the holding of a shift
count. Its size is dependent on the maximum number
of places that a word can be shifted.
An easy way to comprehend the operation of the
control unit is to draw a comparison to a telephone
exchange. The act of dialing a phone number
energizes certain switches and control lines in a
telephone exchange.
In a similar manner, each
program instruction, when executed, causes the
control section to energize certain “switches” and
“control lines.” This enables the computer to perform
the function or operation indicated by the instruction.
A computer program can be stored in the internal
circuits of the computer, or it may be read
instruction-by-instruction from external media. The
internally stored program type of computer is the
most practical type to use when speed and fully
automatic operation are desired. This type of
computer is known as a stored-program computer.
In addition to the command that tells the
computer what to do, the control unit also dictates
how and when each specific operation is to be
performed. It is also active in initiating circuits that
locate information stored in the computer and in
moving this information to the point where the actual
manipulation or modification is to be accomplished.
In the stored-program computer, the control unit
reads an instruction from the memory section. The
information read into the control unit from memory is
in the form of varying voltage levels that make up the
binary word. This word represents a specific
operation that is to be performed. The location of the
data to be operated on is generally a part of the
instruction and energizes circuitry that causes the
specified operation (add, subtract, compare, etc.) to
be executed. Subsequently, the control unit reads the
next instruction or jumps as directed to find the next
instruction to execute.
Computer instructions are broken down into four
general categories.
The instructions are transfer,
arithmetic, logic, and control.
Transfer commands transfer data from one
location to another. One of the instructions is usually
an address in memory, and the other is either a register
or an input/output device.
Arithmetic instructions combine two pieces of
data to forma single piece of data by using one of the
arithmetic operations. In some computers, one of the
pieces of data is in a location specified by an address
contained in an instruction. The other piece of data is
already in a register (usually the accumulator). The
results are usually left in the accumulator.
Logic instructions make the digital computer
much more than an expensive adding machine. The
use of logic instructions enables the programmer to
construct a program capable of a number of tasks.
These instructions enable a computer used for
inventory maintenance to follow one set of
procedures if an inventory item count is too high, and
another if the count is too low. The choice of which
set of procedures to use is made by the control unit
under the influence of the logic instructions. Logic
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